Test linked varinfos in DynamicPPL AD test suite

[penelopeysm]

Follow-up from #890 (comment);

Right now, assuming that #890 gets merged, linking varinfo and attempting to use linked=true in test/ad.jl throws this error:

[ Info: Testing AD on: demo_dot_assume_observe_index_literal - SimpleVarInfo{<:OrderedDict} - AutoReverseDiff(compile=true)
AutoReverseDiff(compile=true): Error During Test at /home/runner/work/DynamicPPL.jl/DynamicPPL.jl/test/ad.jl:62
  Test threw exception
  Expression: DynamicPPL.TestUtils.AD.run_ad(m, adtype; test = true, benchmark = true, varinfo = varinfo, expected_value_and_grad = (ref_logp, ref_grad)) isa Any
  AssertionError: Trying to link already transformed variables
  Stacktrace:
    [1] tilde_assume(::DynamicPPL.DynamicTransformationContext{false}, right::InverseGamma{Float64}, vn::VarName{:s, IndexLens{Tuple{Int64}}}, vi::DynamicPPL.ThreadSafeVarInfo{SimpleVarInfo{OrderedDict{Any, Any}, Float64, DynamicPPL.DynamicTransformation}, Vector{Float64}})
      @ DynamicPPL ~/work/DynamicPPL.jl/DynamicPPL.jl/src/transforming.jl:22
    [2] tilde_assume!!(context::DynamicPPL.DynamicTransformationContext{false}, right::InverseGamma{Float64}, vn::VarName{:s, IndexLens{Tuple{Int64}}}, vi::DynamicPPL.ThreadSafeVarInfo{SimpleVarInfo{OrderedDict{Any, Any}, Float64, DynamicPPL.DynamicTransformation}, Vector{Float64}})
      @ DynamicPPL ~/work/DynamicPPL.jl/DynamicPPL.jl/src/context_implementations.jl:114
    [3] demo_dot_assume_observe_index_literal(__model__::Model{typeof(DynamicPPL.TestUtils.demo_dot_assume_observe_index_literal), (Symbol("##arg#386"),), (), (), Tuple{DynamicPPL.TypeWrap{Vector{Float64}}}, Tuple{}, DefaultContext}, __varinfo__::DynamicPPL.ThreadSafeVarInfo{SimpleVarInfo{OrderedDict{Any, Any}, Float64, DynamicPPL.DynamicTransformation}, Vector{Float64}}, __context__::DynamicPPL.DynamicTransformationContext{false}, arg#386::DynamicPPL.TypeWrap{Vector{Float64}})
      @ DynamicPPL.TestUtils ~/work/DynamicPPL.jl/DynamicPPL.jl/src/test_utils/models.jl:355
    [4] _evaluate!!(model::Model{typeof(DynamicPPL.TestUtils.demo_dot_assume_observe_index_literal), (Symbol("##arg#386"),), (), (), Tuple{DynamicPPL.TypeWrap{Vector{Float64}}}, Tuple{}, DefaultContext}, varinfo::DynamicPPL.ThreadSafeVarInfo{SimpleVarInfo{OrderedDict{Any, Any}, Float64, DynamicPPL.DynamicTransformation}, Vector{Float64}}, context::DynamicPPL.DynamicTransformationContext{false})
      @ DynamicPPL ~/work/DynamicPPL.jl/DynamicPPL.jl/src/model.jl:912
    [5] evaluate_threadsafe!!(model::Model{typeof(DynamicPPL.TestUtils.demo_dot_assume_observe_index_literal), (Symbol("##arg#386"),), (), (), Tuple{DynamicPPL.TypeWrap{Vector{Float64}}}, Tuple{}, DefaultContext}, varinfo::SimpleVarInfo{OrderedDict{Any, Any}, Float64, DynamicPPL.DynamicTransformation}, context::DynamicPPL.DynamicTransformationContext{false})
      @ DynamicPPL ~/work/DynamicPPL.jl/DynamicPPL.jl/src/model.jl:901
    [6] evaluate!!(model::Model{typeof(DynamicPPL.TestUtils.demo_dot_assume_observe_index_literal), (Symbol("##arg#386"),), (), (), Tuple{DynamicPPL.TypeWrap{Vector{Float64}}}, Tuple{}, DefaultContext}, varinfo::SimpleVarInfo{OrderedDict{Any, Any}, Float64, DynamicPPL.DynamicTransformation}, context::DynamicPPL.DynamicTransformationContext{false})
      @ DynamicPPL ~/work/DynamicPPL.jl/DynamicPPL.jl/src/model.jl:831
    [7] link!!(t::DynamicPPL.DynamicTransformation, vi::SimpleVarInfo{OrderedDict{Any, Any}, Float64, DynamicPPL.DynamicTransformation}, model::Model{typeof(DynamicPPL.TestUtils.demo_dot_assume_observe_index_literal), (Symbol("##arg#386"),), (), (), Tuple{DynamicPPL.TypeWrap{Vector{Float64}}}, Tuple{}, DefaultContext})
      @ DynamicPPL ~/work/DynamicPPL.jl/DynamicPPL.jl/src/transforming.jl:34

I need to figure out DynamicTransformationContext a bit more to understand why this happens and why it's not a warning, hence I'm punting it to a future PR, but opening this issue to keep track.

[penelopeysm]

I don't personally see why it can't be a warning here:

DynamicPPL.jl/src/transforming.jl

Lines 15 to 31 in 60ee68e

	function tilde_assume(
	::DynamicTransformationContext{isinverse}, right, vn, vi
	) where {isinverse}
	r = vi[vn, right]
	lp = Bijectors.logpdf_with_trans(right, r, !isinverse)
	if istrans(vi, vn)
	@assert isinverse "Trying to link already transformed variables"
	else
	@assert !isinverse "Trying to invlink non-transformed variables"
	end
	# Only transform if `!isinverse` since `vi[vn, right]`
	# already performs the inverse transformation if it's transformed.
	r_transformed = isinverse ? r : link_transform(right)(r)
	return r, lp, setindex!!(vi, r_transformed, vn)
	end

I changed it to a warning and things happily behave as expected, in that linking a transformed SimpleVarInfo and invlinking an untransformed SimpleVarInfo are both no-ops.

julia> @model f() = x ~ Beta(2, 2)
f (generic function with 2 methods)

julia> s = SimpleVarInfo((x=0.5,))
SimpleVarInfo((x = 0.5,), 0.0)

julia> m = f()
Model{typeof(f), (), (), (), Tuple{}, Tuple{}, DefaultContext}(f, NamedTuple(), NamedTuple(), DefaultContext())

julia> svi = last(DynamicPPL.evaluate!!(m, s, DefaultContext())) # sets logp
SimpleVarInfo((x = 0.5,), 0.4054651081081644)

julia> link(svi, m)
Transformed SimpleVarInfo((x = 0.0,), -0.9808292530117262)

julia> link(link(svi, m), m)
┌ Warning: Trying to link an already transformed variable x
└ @ DynamicPPL ~/ppl/dppl/src/transforming.jl:22
Transformed SimpleVarInfo((x = 0.0,), -0.9808292530117262)

julia> invlink(svi, m)
┌ Warning: Trying to invlink a non-transformed variable x
└ @ DynamicPPL ~/ppl/dppl/src/transforming.jl:24
SimpleVarInfo((x = 0.5,), 0.4054651081081644)

[mhauru]

I've long wondered, in general for all VarInfo types, why we don't make linking/invlinking silent no-ops if there's nothing to change.

[penelopeysm]

I'm not opposed to that, or maybe downgrading the warn to a debug